Chapter 6 Atoms and Starlight

Absorption spectrum dominated by Balmer lines Modern spectra are usually recorded digitally and represented as plots of intensity vs. wavelength.

Emission nebula, dominated by the red Ha line Emission nebula, dominated by the red Ha line

The Balmer Thermometer Balmer line strength is sensitive to temperature: Most hydrogen atoms are ionized => weak Balmer lines Almost all hydrogen atoms in the ground state (electrons in the n = 1 orbit) => few transitions from n = 2 => weak Balmer lines

Measuring the Temperatures of Stars Comparing line strengths, we can measure a star’s surface temperature!

Spectral Classification of Stars Different types of stars show different characteristic sets of absorption lines. Temperature

Spectral Classification of Stars Mnemonics to remember the spectral sequence: Oh Only Be Boy, Bad A An Astronomers Fine F Forget Girl/Guy Grade Generally Kiss Kills Known Me Mnemonics

Stellar spectra O B A F Surface temperature G K M

The Composition of Stars From the relative strength of absorption lines (carefully accounting for their temperature dependence), one can infer the composition of stars.

Infrared spectra of stars Infrared spectra of stars Major differences appear in the infrared spectra of cool stars (M stars ↔ T, L dwarfs).

The Doppler Effect Dl/l0 = vr/c vr The light of a moving source is blue/red shifted by Dl/l0 = vr/c l0 = actual wavelength emitted by the source Dl = Wavelength change due to Doppler effect vr = radial velocity Blue Shift (to higher frequencies) Red Shift (to lower frequencies) vr

Example: Earth’s orbital motion around the sun causes a radial velocity towards (or away from) any star.

Everyday use of Doppler Effect

Astronomical Use

Chapter 7: The Sun

General Properties Average star Spectral type G2 General Properties Average star Spectral type G2 Only appears so bright because it is so close. Absolute visual magnitude = 4.83 (magnitude if it were at a distance of 32.6 light years) 109 times Earth’s diameter 333,000 times Earth’s mass Consists entirely of gas (av. density = 1.4 g/cm3) Central temperature = 15 million 0K Surface temperature = 5800 0K

Very Important Warning: Never look directly at the sun through a telescope or binoculars!!! This can cause permanent eye damage – even blindness. Use a projection technique or a special sun viewing filter

The Photosphere The solar corona Apparent surface layer of the sun Apparent surface layer of the sun Depth ≈ 500 km Temperature ≈ 5800 oK Highly opaque (H- ions) Absorbs and re-emits radiation produced in the solar interior The solar corona

Energy Transport in the Photosphere Energy generated in the sun’s center must be transported outward. In the photosphere, this happens through Convection: Cool gas sinking down Bubbles of hot gas rising up Bubbles last for ≈ 10 – 20 min. ≈ 1000 km

Granulation … is the visible consequence of convection

The Solar Atmosphere Heat Flow Temp. incr. inward Only visible during solar eclipses Apparent surface of the sun Heat Flow Temp. incr. inward Solar interior

The Chromosphere Filaments Region of sun’s atmosphere just above the photosphere. Visible, UV, and X-ray lines from highly ionized gases Temperature increases gradually from ≈ 4500 oK to ≈ 10,000 oK, then jumps to ≈ 1 million oK Filaments Transition region Chromospheric structures visible in Ha emission (filtergram)

Each one lasting about 5 – 15 min. The Chromosphere Spicules: Filaments of cooler gas from the photosphere, rising up into the chromosphere. Visible in Ha emission Each one lasting about 5 – 15 min.

The Layers of the Solar Atmosphere Ultraviolet Visible Sun Spot Regions Photosphere Corona Chromosphere Coronal activity, seen in visible light

Helioseismology The solar interior is opaque (i.e. it absorbs light) out to the photosphere. Only way to investigate solar interior is through Helioseismology = analysis of vibration patterns visible on the solar surface: Approx. 10 million wave patterns!

Energy Production Nuclear Fusion Energy generation in the sun (and all other stars): Binding energy due to strong force = on short range, strongest of the 4 known forces: electromagnetic, weak, strong, gravitational Nuclear Fusion = fusing together 2 or more lighter nuclei to produce heavier ones Nuclear fusion can produce energy up to the production of iron. For elements heavier than iron, energy is gained by nuclear fission.

Energy generation in the Sun: The Proton-Proton Chain Basic reaction: 4 1H → 4He + energy Need large proton speed ( high temperature) to overcome Coulomb barrier (electromagnetic repulsion between protons). 4 protons have 0.048*10-27 kg (= 0.7 %) more mass than 4He. T ≥ 107 0K = 10 million 0K Energy gain = Dm*c2 = 0.43*10-11 J per reaction. Sun needs 1038 reactions, transforming 5 million tons of mass into energy every second, to resist its own gravity.